This invention relates to a marine propulsion system, and more particularly to an outboard marine propulsion system.
In a marine application, it is generally desirable to minimize the frontal area of the submerged drive components in order to reduce drag caused by such components during operation. This presents a difficult problem in high power applications when utilizing a standard perpendicular shaft and bevel gear power transfer arrangement for transferring power from the power head to the propeller. When the proper gear ratio is selected for the optimum propeller in a high power system, the diameter of the bevel gears increases, thereby causing an increase in the transverse dimension of the portion of the gearcase which houses the bevel gears. The resulting increase in gearcase drag certainly detracts from the supposed increase in performance provided by the optimum propeller, and may even result in a decrease in overall performance. That is, the large propeller required for efficiently transferring power through the system requires a gear design furnishing large reduction ratio. Such large reduction ratios are only accomplished by increasing the size of the bevel gears, thereby increasing hydrodynamic drag of the torpedo in which the bevel gears are housed.
If the diameter of the gearcase torpedo is reduced to a more desirable hydrodynamic size, resulting in a reduction in the size of the gears housed therein, then the gear ratio is likewise reduced to the extent that the propeller no longer operates efficiently.
A solution to the above-described problem for a stern drive marine propulsion system is described in copending application Ser. No. 07/244,994 filed 9-15-1988 in the name of Neil A. Newman, and entitled "Stern Drive Marine Propulsion System Including a Chain Drive Mechanism".
The present invention is designed as a solution to the above-noted problems in an outboard application. The invention incorporates an outboard marine propulsion system in which the propeller shaft, disposed in the lower portion of the lower unit, is provided with a sprocket and is driven by a chain. The chain extends between the propeller shaft and an intermediate shaft located above the water line during boat operation. The propeller shaft and the intermediate shaft are substantially parallel, and are oriented so as to be substantially perpendicular to the drive shaft which depends from the power head of the outboard system and is rotatably driven by an engine. A first drive means is disposed between the depending drive shaft and the intermediate shaft for driving the intermdiate shaft in response to rotation of the drive shaft. A second drive means, comprising the above-mentioned chain means, is disposed between the intermediate shaft and the propeller shaft. In a preferred embodiment, the first drive means comprises a bevel gear arrangement for transferring power from the depending drive shaft to the intermediate shaft. A reverse gear and a forward gear are mounted to the depending drive shaft, and are engageable with a bevel gear provided on an end of the intermediate shaft. With the intermediate shaft being located above the water line, the bevel gears transferring power from the drive shaft to the intermediate shaft can be of a relatively large size so as to provide the large reduction ratios required in a high power system while providing no adverse effect on drag of the lower unit caused by the submerged components. A clutch mechanism is mounted to the depending drive shaft for selectively engaging either the forward or reverse gear for imparting rotation to the intermediate shaft in a desired rotational direction. The chain extending between the intermediate shaft and the propeller shaft is preferably housed in a pair of substantially vertical struts formed in the lower unit housing. The struts are preferably spaced from each other so as to provide an air gap therebetween, for reducing the frontal area of the submerged components of the lower unit.